A tip to help you is to go back to your computer and re download the Ocarine Program and put the codes on the SD card again and go try it again in your Wii. If everything is working correctly then congrats, and go have fun with your hacks!!! If everything is not okay then sadly I have no suggestions(This did not happen to me). Now go to the Homebrew Channel and open the Ocarine Program and make sure everything works.
When you have finished that take the SD card out and put it in your wii along with the Mario Kart Wii game. Now that you have the Homebrew channel you need to download Ocarina program.Once you have that hook up your SD card to your computer and download all the hacks you want onto the card using the Ocarine program. These games will not help you get hacks, but are fun to play!!! Once you have the Homebrew Channel you can dowload free games. I suggest you do not buy the Twilight game for you will not need it later. I would like to warn you all that you can only get banned if you are online and hacking at the same time, breaking the NINTENDO user agreement! Whatever you do outside their network is your business unless you use non Nintendo hardware or software to do it, or you modify the system hardware to do it, thus breaking the warranty agreement! Now that you have been warned about what may happen here is what you will need: I'm so close to getting hacks and wanted to share more info I got about becoming a hacker. And make sure you put “Infinite” before the power you want (the most fun powers are Star, Mega Mushroom, Triple Red Turtle Shells, Triple Green Turtle Shells, Bullet Bil l(the rocket) and some time Triple Banana, Fake Item Box and Bomb-omb.) OK? Now type the codes in the code box and press start.You will be able to hack Mario Kart on the Homebrew Channel.My guess to Stop hacking Mario Kart is to take your SD Card out of your Wii. Type in as many as you want but make sure you call it what it is called in the Mario Kart Wii Instructions Manual. Then select rmce01 is Mario Kart Wii's ID. Use Legend of the Twilight Princess game to get Homebrew Channel and then
Update (6/19/09): from reader Rosalina, you can indeed hack it from the homebrew channel: I don't know the specifics, so feel free to leave a comment if you know any more than that.
If you install the homebrew channel, you can enable cheats from there. Update: looks like it's easier to cheat nowadays.